1. Field of the Invention
The present invention relates to a power supply device having an overcurrent protection function, and more particularly to a power supply device for supplying drive power to an external peripheral device, which has an overcurrent protection function for blocking drive power from being supplied from the power supply device to the peripheral device when an overcurrent is applied to the peripheral device, thereby preventing in advance the power supply device and the peripheral device from being damaged by the overcurrent.
2. Description of the Related Art
As shown in FIG. 1, a first conventional power supply device includes a power supplier 10, a power interrupter 20 and a microcomputer 30. The power supplier 10 supplies drive power to an external peripheral device. The power interrupter 20 turns the power supplier 10 on/off to determine whether to supply drive power to the peripheral device. The microcomputer 30 is connected to the power interrupter 20 to generate a control signal for turning the power interrupter 20 on/off.
The power supply device further includes a power supply source 40 and a power output terminal 50. The power supply source 40 supplies the drive power to the power supplier 10. The power output terminal 50 is connected to the peripheral device so as to allow the drive power passing through the power supplier 10 to be supplied to the peripheral device.
The power supplier 10 includes a first transistor Q1 and a resistor R1. The first transistor Q1 has its emitter connected to the power supply source 40 and its collector connected to the power output terminal 50. The resistor R1 is connected between the emitter and the base of the first transistor Q1.
The power interrupter 30 includes a second transistor Q2 and a resistor R3. The transistor Q2 has its collector connected to the base of the first transistor Q1 through a resistor R2. The resistor R3 is connected between the base and the emitter of the second transistor Q2. The base of the second transistor Q2 is connected to the microcomputer 30 through a resistor R4.
The conventional power supply device having such a configuration operates in the following manner.
First, the microcomputer 30 generates a control signal for controlling the power interrupter 20, and the generated control signal is inputted to the base of the second transistor Q2, thereby turning the second transistor Q2 on.
As the second transistor Q2 is turned on, the first transistor Q1 of the power supplier 10 is turned on. Accordingly, the drive power supplied from the power supply source 40 is transferred to the power output terminal 50 through the first transistor Q1, which is thus supplied through the power output terminal 50 to an external peripheral device connected to the terminal 50.
However, since it has no function to prevent an overcurrent from being applied to the peripheral device, the conventional power supply device has a problem in that the overcurrent may damage not only the peripheral device but also the power supply device.
A device for preventing the overcurrent has been proposed in Japanese Patent Publication No. 1996-317545, which is shown in FIG. 2. In this device, the magnitude of a voltage applied to a second transistor 62 is measured based on the respective resistances of first and second resistors 71 and 72, provided in a resistor portion 70 connected in parallel to first and second transistors 61 and 62, and two resistors 63 and 64 in power switching means 60. A controller 80 turns the second transistor 62 off if the measured magnitude is equal to or more than a predetermined level.
In this manner, a separate circuit (or controller) may be used to block drive power supplied from a power supply device from being transferred to a peripheral device when an overcurrent is applied to the peripheral device, so as to prevent the overcurrent from being applied to the peripheral device. However, the necessity to provide additional parts for forming the separate circuit leads to an increase in the cost, and also decreases the reliability due to errors in the additional parts.